1. Field of the Invention
The present invention relates to a projection system and an optical path transfer device thereof, and particularly to a projection system and an optical path transfer device thereof with high illumination efficiency.
2. Description of the Related Art
In a conventional optical projection display, a light beam emitted from a light source is projected toward a digital micromirror device (DMD). The DMD is formed with a plurality of micromirrors selectively disposed in one of two predetermined tilt angles, in which the light beam is guided and reformed for projection as an image onto the display screen. Thus, each of the pixels on the display can be selectively switched to either a bright mode, in which the light beam passes through the projection system, or a dark mode, in which the light beam does not pass through the projection system.
FIG. 1 illustrates a conventional projection system 100. FIG. 2A is a partial perspective view of the total internal reflection prism (TIR prism) 112 and the DMD 114 in FIG. 1. Further, a side view of the X-Y plane in FIG. 2A is shown in FIG. 2B, and a top view of the X-Z plane in FIG. 2A is shown in FIG. 2C.
The optical path of the projection system 100 is described hereinafter with reference to FIG. 1. The light beam I, emitted from the light source 102 and condensed by the reflector 104, passes through the color wheel 106, the light tunnel 108, the relay lens 110 and the total internal reflection prism (TIR prism) 112, and is projected toward the DMD 114. With the switching of the DMD 114, the light beam I corresponding to the image signal passes through the projection lens 116 and is projected toward the display device 118 to display the image.
Generally, the projection lens 116 and the DMD 114 are disposed in an on-axis configuration. Specifically, the light beam I is directed toward the projection lens 116 along a direction substantially parallel to the light axis of the projection lens 116. Accordingly, the light beam I reflected by the TIR prism 112 is projected toward the DMD 114 at a tilt angle θ. That is, the light beam I is projected toward the DMD 114 in an off-axis manner with the tilt angle θ.
Configuration of the optical path can be further described in detail with reference to FIG. 2A to FIG. 2C. In FIG. 2A, the light beam I is projected perpendicularly toward the TIR prism 112 on the incident surface 120 and is reflected toward the DMD 114 in the off-axis manner. In view of the X-Y plane as shown in FIG. 2B, the light beam I is incident on the DMD 114 at a tilt angle θ.
Further, in view of the X-Z plane as shown in FIG. 2C, the light beam I in the TIR prism 112 is directed in a direction parallel to two side surfaces 122 of the TIR prism 112.
Since the light beam I from the TIR prism 112 is incident on the DMD 114 at the tilt angle θ, an illumination area 126 of the light beam I on the DMD 114 is stretched and deformed as a parallelogram. However, the illumination area 126 may exceed the active area 206, generally in a rectangular shape, of the DMD 114. Accordingly, luminosity waste occurs in the DMD 114, which reduces the illumination efficiency of the projection system 100.